Grains containing fats and oils have been dried by microwave heating followed by steps to remove husks and to extract oils, e.g. see U.S. Pat. No. 4,464,402, Aug. 7, 1984, Gannon. Grains and seeds also have been extracted using microwave rays to heat the extractant medium: see Ganzler & Salgo, 1987, Z. Lebensm Unters Forsch 184: 274-276. In the experiments of Ganzler & Salgo, the radiation served strictly to heat the medium (repeated cooling necessary) and very little could reach the grains and seeds. Microwave drying of other food products followed by solvent extraction has been used as an analytical procedure, e.g. U.S. Pat. No. 4,554,132, Nov. 19, 1985, Collins. In U.K. Patent 1,209,675, Oct. 21, 1970, Byrne, palm fruits are heated by microwave sufficiently to inactivate enzymes followed by a solvent extraction of palm oil. Canadian Patent 987,993, Apr. 27, 1976, Heitkamp et al, describes a microwave-induced migration of flavour and aroma constituents toward the surface in plant tissues such as tobacco or tea in the presence of moisture and optionally a solvent. The flavour or aroma was enhanced when subsequently the tobacco or tea was used. There was no mention of enhanced extraction of components into an extractant: the microwave dose and amounts of solvent were too low for this to have occurred. Plant material has been exposed to microwave in an air stream thus producing a head-spacelike sample of volatile material: see A. A. Craveirs et al, 1989, Flavour and Fragrance Journal 4: 43-44.
The need for a general extraction method that can be used for plant material from a variety of origins is well recognized. The food industry, in particular, requires methods that are versatile, relatively inexpensive to perform and that do not involve intricate operations that increase the risks of failure and of health hazards for both the plant personnel and the consumers.